Three-piece, wound golf balls with balata covers are preferred by many expert golfers. These balls provide a combination of distance, high spin rate, and control that is not available with other types of golf balls. However, balata is easily damaged in normal play, and lacks the durability required by the average golfer.
Thus, over the past several years, golf ball manufacturers have been using ionomer resins for golf ball cover materials because of their durability, rebound, and scuff resistance characteristics. Ionomer resins are generally understood as copolymers of an olefin and an α,β-unsaturated carboxylic acid, e.g., acrylic acid, methacrylic acid, or maleic acid, wherein the acidic groups are partially neutralized with metal ions such as sodium, lithium, zinc, or magnesium ions. Ionomer-covered balls are impossible to “cut”, but also have a very hard “feel”, which many golfers find unacceptable. In addition, ionomer-covered golf balls generally have a lower spin rate (attributed to the differences in the composition and construction of both the cover and the core), which makes these type of balls more difficult to draw or fade.
As such, many attempts have been made to produce a golf ball with the control and feel of a wound balata ball and the durability of a solid, two-piece ball, but none have succeeded totally. In various attempts to produce an ideal golf ball, the golfing industry has blended hard ionomer resins (i.e., those ionomer resins having a hardness of about 60 Shore D to about 66 Shore D, as measured in accordance with ASTM method D-2240) with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally led to numerous processing problems and result in golf balls limited to shorter distance play. For example, whereas blends of one variety of polymer, such as ionomers, have been successfully used, blends of one type of polymer with other non-ionic polymers are typically immiscible, i.e., heterogeneous on a microscopic scale, and incompatible, i.e., heterogeneous on a macroscopic scale, unless strong interactions are present between the polymer components in the mixture. These strong interactions include those observed between carboxylic acid based ionomers and other polymers containing carboxylic acid groups.
In particular, this lack of compatibility exists when an ionomer is blended with a polyolefin homopolymer, copolymer, or terpolymer that does not contain ionic, acidic, basic, or other polar pendant groups, and is not produced with a metallocene catalyst. These mixtures often have poor tensile strength, impact strength, and the like. Hence, the golf balls produced from these incompatible mixtures will have inferior golf ball properties such as poor durability, cut resistance, and the like. In contrast, a compatible blend may be heterogeneous on a microscopic scale, but is homogeneous on a macroscopic scale, and, thus, has useful golf ball properties.
In this regard, U.S. Pat. No. 5,397,840 discloses golf ball covers including a blend of “ionic copolymers” and “non-ionic copolymers”. However, the “ionic copolymers” are defined as copolymers of an α-olefin and a metal salt of an α,β-carboxylic acid, and the “non-ionic copolymers” are copolymers or terpolymers containing ethylene or propylene and acrylic or methacrylic acid monomers. Therefore, strong interactions exist between the metal salts of the “ionic copolymers” and the acrylic or methacrylic acid monomers of the “non-ionic copolymers” that allow compatible blends to be formed. These interactions do not exist in prior art blends of ionomers and polymers that are truly non-ionic or non-polar.
In addition, U.S. Pat. No. 5,616,640 to Harris et al. discloses golf ball cover compositions including an oxa acid compound having the formula:
which may be blended with carboxylic acid based ionomers to provide golf balls having an excellent spin rate and good shear resistance. Moreover, U.S. Pat. Nos. 5,869,578 and 6,255,361 are directed to golf balls including saponified ionomers and compatible blends of oxa acids and saponified ionomers.
Those of ordinary skill in the art are aware that increasing the neutralization of ethylene-based ionomers during manufacturing, however, reduces the processability of the material. This is demonstrated by the decreased melt flow index of the resulting material. In fact, in some cases, the melt flow index of the material is decreased to the point that the material does not flow at all. As a result, commercially available ethylene-based ionomers are generally only partially neutralized.
And, while highly neutralized polymers have recently been discussed in U.S. Pat. No. 6,329,458, U.S. Patent Publication Nos. 2001/0019971, 2001/0018375, 2003/0013549, and International Publication No. WO 01/29129, these polymers are produced using fatty acids. In fact, DuPont recently released a highly neutralized polymer produced from a fatty acid under the trade name DuPont® HPF 1000 (formerly known as DPO AD1016-2). Potential compatibility issues remain with these highly neutralized polymers, however, due to their hydrophobic backbone moiety. In addition, the non-neutralized fatty acids may vaporize during injection molding, which may cause molding defects and defects during post-processing, e.g., during painting, coating, and the like.
Thus, a need exists in the golf ball art for a resin material that is easily processed with desirable melt flow and molding characteristics. In addition, a need exists in the art for a method to mold this resin material into highly durable golf balls with improved performance and tailorable to have virtually any combination of feel and spin rate.